An inherent problem with pressurized tennis balls is their loss of internal pressure after being removed from pressurized storage containers. Once exposed to the pressure of the earth's atmosphere, the internal pressure drops due to the diffusion of air through the wall of the tennis ball center, i.e., the core. When the pressure drops from the initial approximately 15 psi to about 10 psi and/or when the rebound thereof drops from the original approximately 55 percent rebound to less than 53 percent, changes take place in the play of the ball which are undesirable and often unacceptable.
Heretofore, long life pressurized tennis balls used coatings or films applied to the surface as a barrier layer for the core, substituted special low permeable rubbers for the accepted natural rubber cores, or contained special additives in the rubber core. Another method was to use special gasses in the core which permeated through the tennis ball cores more slowly than air.
Generally, none of these approaches have been commercially effective. For example, poly(vinylidene chloride) has been utilized as a barrier layer, but upon being struck by a racquet, the barrier layer would rupture and diffusion of air would take place more rapidly. Substituted rubber core materials included butyl rubber. However, the relatively thick walls required to effectively reduce air permeation adversely affected the rebound of tennis balls to unacceptable levels. Special low permeable gasses such as mixtures of air with SF.sub.6 and CF.sub.4 while producing a long life tennis ball are characterized by high cost, and having an audible and undesirable "ping" when the ball is struck by a racquet.